E ﬀ ects of Nutrient Solution Irrigation Quantity and Downy Mildew Infection on Growth and Physiological Traits of Greenhouse Cucumber

: Abiotic and biotic stresses both decrease the quality and quantity of cultivated plants. In this study, in order to see the responses of cucumber plants to drought stress and cucumber downy mildew infection, downy mildew infestation at different two levels, B1 (disease infestation) and B2 (no disease infestation), along with three fertigation requirement levels, full fertigation T1, moderate nutrient solution deficit T2 and severe nutrient solution deficit T3, were applied in a greenhouse. Thus, six treatments, i.e., B1T1, B1T2, B1T3, B2T1, B2T2 and B2T3, were set. The leaf gas-exchange parameters were signiﬁcantly increased under CK (control experiment, B2T1: no disease infestation and full irrigation) treatment, and leaf photosynthesis rate, transpiration rate and stomatal conductance were signiﬁcantly decreased under the B1T1 treatment. Leaf intercellular CO 2 concentration was signiﬁcantly increased under B1T1 treatment. Leaf photosynthesis rate, transpiration rate, intercellular CO 2 concentration and stomatal conductance were signiﬁcantly decreased under B1T2, B1T3, B2T2 and B2T3 treatments. Compared with treatment CK (B2T1), the plant height of cucumber under B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 11.41%, 19.05%, 27.48%, 7.55% and 10.62%, respectively; the stem diameter of cucumber under B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 5.70%, 13.45%, 23.03%, 9.46% and 15.74%, respectively; and leaf area of cucumber under B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 22.79%, 38.68%, 58.28%, 13.76% and 29.96%, respectively. The root–shoot ratio of cucumber under B1T1, B1T2, B1T3, B2T1, B2T2 and B2T3 treatments was 3.16%, 2.99%, 4.11%, 3.92%, 3.13% and 3.63%, respectively. The root–shoot ratio of cucumber was the highest under the B1T3 treatment.


Introduction
With the continuously increasing global population, the demand for food production has increased [1][2][3]. Greenhouses are conducive to the growth of vegetables and are hardly affected by the external environment, which plays an important role in improving vegetable yield [4,5]. At present, China's greenhouse cultivation area measures more than 4 million hectares [6,7]. Cucumber, as an important economic crop, is one of the main vegetables widely planted in the world. It has a status second only to tomato in fruit vegetables [8,9]. Because of its rich taste and nutritional value, cucumber is deeply loved by consumers and plays an increasingly important role in agricultural

Study Site and Treatment Details
The experiment was performed in a Venlo-type greenhouse at the Key Laboratory of Modern Agricultural Equipment and Technology, Ministry of Education, Jiangsu University, Zhenjiang, China (119 • 45 E, 32 • 20 N). The average greenhouse air temperature was 24.5 • C (the range was from 14.57 to 38.69 • C). The relative humidity of the greenhouse was 82.6%. The experiment was conducted from 24 August 2020 to 27 September 2020.
The cucumber seeds, "Jinyou 1" (provided by the cucumber research institute in Tianjin Academy of Agricultural Sciences, Tianjin, China), were sowed in a plug tray on 24 August 2020. Plastic pots (having dimensions of 20.9 cm in height and 32 cm in diameter) were filled with perlite substrate (the perlite substrate in the plastic pots was washed with running water), used as a growing medium for the plants, on 30 August 2020. Cucumber seedlings were transplanted into plastic pots on 31 August 2020. Seedlings planting density was 5.54 plants/m 2 . Full fertigation was applied in order to ensure growth of cucumber seedlings. Cucumber seedlings were roped by a nylon cord vertically. Pruning was done to maintain the cucumber plants' growth by following agronomic Agronomy 2020, 10,1921 3 of 13 requirements. The cucumbers were picked at the flowering stage (70% of the cucumbers were in bloom) on 27 September 2020.

Treatments and Experiment Design
To investigate the effect of downy mildew infestation and nutrient solution irrigation (Kawasaki nutrient solution; the composition of the nutrient solution is shown in Supplementary Table S1) on crop growth, leaf gas-exchange, material accumulation (including leaf fresh weight, leaf dry weight, stem fresh weight, stem dry weight, root fresh weight and root dry weight of cucumber plants) and parameters of root systems of cucumber plants, as shown in Figure 1, the experimental design consisted of 6 treatments, each of which was repeated 6 times. Downy mildew infestation at different two levels, B1 (disease infestation) and B2 (no disease infestation), along with three fertigation requirement levels, full fertigation T1, moderate nutrient solution deficit T2 and severe nutrient solution deficit T3, were applied in the greenhouse. Thus, six treatments, i.e., B1T1 (disease infestation and full fertigation), B1T2 (disease infestation and moderate nutrient solution deficit), B1T3 (disease infestation and severe nutrient solution deficit), B2T1 (no disease infestation and full fertigation), B2T2 (no disease infestation and moderate nutrient solution deficit) and B2T3 (no disease infestation and severe nutrient solution deficit), were set. T1, T2 and T3 were 600 (field water capacity), 400 and 200 mL during the whole experiment period, respectively. The nutrient solution was irrigated at 8:00-9:00 in the morning. The cucumber was treated with different nutrient solution irrigation treatments and downy mildew infection on the 9th day after transplanting.

Treatments and Experiment Design
To investigate the effect of downy mildew infestation and nutrient solution irrigation (Kawasaki nutrient solution; the composition of the nutrient solution is shown in Supplementary Table S1) on crop growth, leaf gas-exchange, material accumulation (including leaf fresh weight, leaf dry weight, stem fresh weight, stem dry weight, root fresh weight and root dry weight of cucumber plants) and parameters of root systems of cucumber plants, as shown in Figure 1, the experimental design consisted of 6 treatments, each of which was repeated 6 times. Downy mildew infestation at different two levels, B1 (disease infestation) and B2 (no disease infestation), along with three fertigation requirement levels, full fertigation T1, moderate nutrient solution deficit T2 and severe nutrient solution deficit T3, were applied in the greenhouse. Thus, six treatments, i.e., B1T1 (disease infestation and full fertigation), B1T2 (disease infestation and moderate nutrient solution deficit), B1T3 (disease infestation and severe nutrient solution deficit), B2T1 (no disease infestation and full fertigation), B2T2 (no disease infestation and moderate nutrient solution deficit) and B2T3 (no disease infestation and severe nutrient solution deficit), were set. T1, T2 and T3 were 600 (field water capacity), 400 and 200 mL during the whole experiment period, respectively. The nutrient solution was irrigated at 8:00-9:00 in the morning. The cucumber was treated with different nutrient solution irrigation treatments and downy mildew infection on the 9th day after transplanting.

Extraction and Inoculation of Pathogenic Fungi
In May 2020, cucumber downy mildew sporangiospores were collected in the Venlo-type greenhouse of the Key Laboratory of Modern Agricultural Equipment and Technology of the Ministry of Education of Jiangsu University, China. First, we rinsed the aged spores and bacteria on the plants' leaves, which were cleaned with sterile water. The leaves were placed on a potato dextrose agar (PDA; Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) medium. When the PDA medium was covered with spore colonies, we used sterile inoculation needles to select different colonies and inoculated them on a PDA medium containing cucumber leaves until only one colony was growing on the PDA medium. Then, the colonies on the PDA medium were rinsed with sterile water to prepare a sporangia or conidia suspension. Finally, the morphology of spores was observed under an ultra-deep three-dimensional microscope (VHX-900F, made by KEYENCE Co., Osaka, Japan) with a cell-counting plate, and pathogens were screened out according to relevant data. Then, the extracted cucumber downy mold spore was subcultured on cucumber leaves (as shown in Supplementary Figure S1) [27]. When the fifth leaf of cucumber was grown (8 September 2020),

Extraction and Inoculation of Pathogenic Fungi
In May 2020, cucumber downy mildew sporangiospores were collected in the Venlo-type greenhouse of the Key Laboratory of Modern Agricultural Equipment and Technology of the Ministry of Education of Jiangsu University, China. First, we rinsed the aged spores and bacteria on the plants' leaves, which were cleaned with sterile water. The leaves were placed on a potato dextrose agar (PDA; Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) medium. When the PDA medium was covered with spore colonies, we used sterile inoculation needles to select different colonies and inoculated them on a PDA medium containing cucumber leaves until only one colony was growing on the PDA medium. Then, the colonies on the PDA medium were rinsed with sterile water to prepare a sporangia or conidia suspension. Finally, the morphology of spores was observed under an ultra-deep three-dimensional microscope (VHX-900F, made by KEYENCE Co., Osaka, Japan) with a cell-counting plate, and pathogens were screened out according to relevant data. Then, the extracted cucumber downy mold spore was subcultured on cucumber leaves (as shown in Supplementary Figure S1) [27]. When the fifth leaf of cucumber was grown (8 September 2020), cucumber downy mold spores were prepared into a spore suspension of 1 × 10 6 spores/mL with sterile water. Cucumber leaves were inoculated in the evening with a disposable sterile syringe with a capacity of 2 mL (each leaf was inoculated with 2 mL spore suspension), and sterile water was set as the control [28].

Leaf Gas-Exchange Parameters
Leaf gas-exchange parameters consisting of leaf photosynthesis rate, transpiration rate, intercellular CO 2 concentration and stomatal conductance were measured using a portable photosynthesis system (LI-6400, LI-COR Inc., Lincoln, NE, USA). The disease index of cucumber downy mildew was graded according to GB/T17980. . The severity of cucumber downy mildew grading criteria is shown in Supplementary Table S2.
The measurements were taken on the 9th (asymptomatic), 14th (diseased area accounts for 6% to 10% of the leaf area) and 19th (diseased area accounts for 26% to 50% of the leaf area) day after the seedlings were transplanted into plastic pots. Each selected date for measurement of leaf gas-exchange was parallel to the leaf disease degree. The measurements were taken on sunny days at 9:00-11:00 local time. The position of the selected leaf was kept similar in each plant.

Crop Growth Parameters and Material Accumulation
Plant height (cm, from perlite substrate surface to cucumber plant top) and stem diameter (mm, at marked point of 10 cm height from perlite substrate surface) of the cucumber plants were measured using a metric ruler and vernier caliper, respectively. The leaf area of each plant was measured after the experiment. In this study, we used a protective cleaning method to wash and tidy the root system; then, chose the Perfection V700 photo scanner (made by EPSON Co., Nagano, Japan) and the WinRHIZO root analysis software (professional version) to measure the root system parameters, including total length, average diameter, surface area, total volume and total tip number. The fresh weights of the leaves, stems and roots of each plant were measured at the end of the experiment. Then, they were put in an oven which was set at 105 • C for 15 min, after which the temperature would be reset to 80 • C, followed by keeping them drying until the weight was constant; a precision electronic scale (0.0001 g) was used to measure the dry weight of leaves, stems and roots [1]. Finally, the root-shoot ratio for the crops was calculated according to the following formula (1): where DW (leaves) represents the dry weight of leaves, DW (stem) represents the dry weight of stems, and DW (root) represents the dry weight of roots.

Statistical Analyses
Data were analyzed using SPSS 16 (SPSS Inc., Chicago, IL, USA) analysis of variance (ANOVA) to study the combined effects of downy mildew infestation and nutrient solution irrigation. The least significant difference (LSD) test was used to determine significance at a significance level of p < 0.05.

Leaf Gas-Exchange Parameters
The effects of different nutrient solution irrigation treatments and different stages after downy mildew infection on (9, 14 and 19 days after transplanting) gas-exchange parameters of cucumber leaves Agronomy 2020, 10, 1921 5 of 13 are shown in Figure 2. As we can see from Figure 2, the leaf gas-exchange parameters were significantly increased under CK (B2T1) treatment, and leaf photosynthesis rate, transpiration rate and stomatal conductance were significantly decreased under the B1T1 treatment. However, leaf intercellular CO 2 concentration was significantly increased under the B1T1 treatment during the whole experiment period. Leaf photosynthesis rate, transpiration rate, intercellular CO 2 concentration and stomatal conductance were significantly decreased under the B1T2, B1T3, B2T2 and B2T3 treatments during the whole experiment period. There was no significant difference found in the gas-exchange parameters of cucumber leaves when measured on the 9th day (that is, before different nutrient solution irrigation treatments and downy mildew infection) after transplanting.
Agronomy 2020, 10, x FOR PEER REVIEW 5 of 13 and B2T3 treatments during the whole experiment period. There was no significant difference found in the gas-exchange parameters of cucumber leaves when measured on the 9th day (that is, before different nutrient solution irrigation treatments and downy mildew infection) after transplanting.   29.27% and 31.07%, respectively. After downy mildew infects cucumber plants, the hyphae enter the tissue through the stomata. In this process, the stomata shrink due to damage, and the stomata conductance decreases. Outside CO2 cannot enter through these deformed stomata, which limits the transmission of CO2 to chloroplasts, resulting in the decrease in its photosynthetic rate and the increase in its intercellular CO2 concentration [27]. With the aggravation of drought degree, the photosynthetic rate,   29.27% and 31.07%, respectively. After downy mildew infects cucumber plants, the hyphae enter the tissue through the stomata. In this process, the stomata shrink due to damage, and the stomata conductance decreases. Outside CO 2 cannot enter through these deformed stomata, which limits the transmission of CO 2 to chloroplasts, resulting in the decrease in its photosynthetic rate and the increase in its intercellular CO 2 concentration [27]. With the aggravation of drought degree, the photosynthetic rate, stomatal conductance and intercellular CO 2 concentration of cucumber leaves decreased gradually. It shows that drought stress is not conducive to the synthesis of organic matter in cucumber plants, and at the same time, it hinders the transportation of water and organic matter. Compared with water stress treatment alone, downy mildew infection and water stress treatment had greater effects on the photosynthetic rate, stomatal conductance and intercellular CO 2 concentration of cucumber leaves. The results of this experiment are consistent with previous studies [23,29].
From Figure 2b, it can be seen that at 14 days after transplanting, compared with treatment B2T1, the intercellular CO 2 concentration of leaves under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 1.21%, 17.19%, 20.31%, 29.41% and 34.61%, respectively. Additionally, at 19 days after transplanting, compared with treatment B2T1, the intercellular CO 2 concentration of leaves under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 0.63%, 28.75%, 32.22%, 39.94% and 43.67%, respectively. After downy mildew infected the cucumber plants, the intercellular CO 2 concentration of cucumber leaves increased gradually [29]. Therefore, there was no significant difference found in intercellular CO 2 concentration parameters of cucumber leaves under B1T1 and B2T1 treatments when measured 14 and 19 days after transplanting, respectively. Different from downy mold infection, with the aggravation of drought degree, the intercellular CO 2 concentration of cucumber leaves decreased gradually [30]. However, when downy mildew infection and drought stress occurred simultaneously, it can be seen from Figure 2b that drought stress had a greater impact on the intercellular CO 2 concentration of cucumber leaves.
From Figure 2d, it can be seen that at 14 days after transplanting, compared with treatment B2T1, the transpiration rate of leaves under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 20.81%, 30.63%, 40.74%, 26.17% and 34.42%, respectively, and at 19 days after transplanting, compared with treatment B2T1, the transpiration rate of leaves under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 40.47%, 50.03%, 57.36%, 38.52% and 44.38%, respectively. Cucumber leaves infected with downy mildew will increase the water loss of the leaves by increasing the transpiration rate. There are some differences between the results of this study and those of a related study [23]. The inconsistency between the results of this study and their results may be due to the short research period of this experiment. At the same time, these are also affected by many environmental factors and agricultural practices, such as atmospheric CO 2 concentration, light incidence, nutrient solution irrigation and agricultural tillage [31][32][33].

Cucumber Plants Growth Parameters
The influence of different nutrient solution irrigation treatments and downy mildew infection on the plant height, stem diameter and leaf area of cucumber is shown in Table 1 drought stress occurred simultaneously. From the above analysis, we can know that when drought stress occurred or downy mildew infection and drought stress occurred at the same time, the effect on cucumber leaf area was the greatest, while the effect on the cucumber stem diameter was the weakest. The results of plant height in this experiment are consistent with previous studies. The results of stem diameter and leaf area, however, are slightly different [21,34]. The reason may be that previous studies mainly focused on abiotic stress or a single biological stress. The experimental conditions were different [28,35,36]. In addition, it may also be related to the different cucumber varieties studied. Previous studies mainly focused on the effect of downy mildew infection on the disease-resistant and non-disease-resistant varieties of cucumber [37].

Parameters of Root Systems
The influence of different nutrient solution irrigation treatments and downy mildew infection on the root system parameters of cucumber are shown in Table 2. Under different treatment conditions, there were significant differences (p < 0.05) in the total length of root, the surface area of root and the total tips of root. Compared with treatment CK (B2T1), the total length of root under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 32.98%, 42.96%, 48.62%, 9.62% and 21.54%, respectively; the surface area of root under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 49.97%, 52.07%, 52.75%, 9.04% and 26.39%, respectively; the average diameter of root under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 46.51%, 56.17%, 62.38%, 19.07% and 28.28%, respectively; the total volume of root under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 56.36%, 59.63%, 64.03%, 11.69% and 31.21%, respectively; the total tips of root under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 48.72%, 53.92%, 59.87%, 16.55% and 28.10%, respectively. From the above analysis, we can know that the total length, surface area, average diameter, total volume and total tips of the root showed different responses under different nutrient solution irrigation treatments and downy mildew infection, and unlike drought stress, there is no significant difference (p > 0.05) in surface area, average diameter and total volume of root when downy mildew infection and drought stress occur simultaneously. The results are different from total length and total tips of root in this experiment. Although we used a protective cleaning method to wash and tidy the root system, experimental errors may also occur when root washing is performed [1,38].

Cucumber Plant Parameters
The effects of nutrient solution irrigation treatments and downy mildew infection on the biomass of cucumber plants are shown in Figure 3. As we can see from Figure 3, the biomass of cucumber plants showed different responses under different nutrient solution irrigation treatments and downy mildew infection. Compared with treatment CK (B2T1), the fresh weight of cucumber leaves under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 17.1%, 27.75%, 46.28%, 13.74% and 30.97%, respectively; the dry weight of cucumber leaves under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 18.52%, 32.80%, 55.03%, 11.77% and 25.77%, respectively; the fresh weight of cucumber stem under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 23 Figure 3 that there was no significant difference (p > 0.05) in the fresh weight and dry weight of cucumber leaves or in the fresh weight and dry weight of cucumber stems under the B1T1, B1T2 and B2T3 treatments, and there was no significant difference (p > 0.05) in the fresh weight and dry weight of cucumber roots under the B1T1 and B1T2 treatments.
The photosynthetic rate and formation rate and the amount of assimilate decreased when the plants were infected by pathogens and under water-deficit stress. In order to adapt to the new living environment, on the one hand, plants use limited assimilates to synthesize some substances to maintain certain permeability of cells and tissues to adapt to pathogen infection and drought environment, on the other hand, they continue to provide other cells and tissues to maintain a certain growth rate. Pathogen infection and drought inhibited the outward transportation of sucrose from the mesophyll cells but promoted the loading of the phloem, while the long-distance transportation of sucrose was not affected. Pathogen infestation and drought lead to a decrease in the rate of assimilation formation and the preferential distribution of assimilation to synthetic osmotic adjustment substances will inevitably affect the root-shoot ratio of plants. The influence of different nutrient solution irrigation treatments and downy mildew infection on the root-shoot ratio of cucumber is shown in Figure 4.
As we can see from Figure 4, the root-shoot ratio of cucumber showed different responses under different nutrient solution irrigation treatments and downy mildew infection. The root-shoot ratio of cucumber under the B1T1, B1T2, B1T3, B2T1, B2T2 and B2T3 treatments was 3.16%, 2.99%, 4.11%, 3.92%, 3.13% and 3.63%, respectively. The root-shoot ratio of cucumber was the highest under the B1T3 treatments. When plants are infected by pathogenic and water-deficit stress, the degree of inhibition of root growth is much lower than that of aboveground growth. Therefore, limited assimilation is preferentially distributed to the root system, resulting in an increase in root-shoot ratio [21,39]. Leaf water status is severely affected by pathogenic and water-deficit stress at the cellular levels and encompasses disruptions in plant metabolism. On the other hand, due to the destruction of the leaf cuticle and increase in cell membrane permeability, water loss from the infected leaf areas was increased in plants which were infected by pathogens [40]. Therefore, growth is inhibited under the stress of water and pathogenic bacteria, which may be due to the increase in respiration rate and the decrease in photosynthetic activity, resulting in a decrease in total biomass. cucumber root under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 36.94%, 50.17%, 52.34%, 28.63% and 34.04%, respectively. It can be seen from Figure 3 that there was no significant difference (p > 0.05) in the fresh weight and dry weight of cucumber leaves or in the fresh weight and dry weight of cucumber stems under the B1T1, B1T2 and B2T3 treatments, and there was no significant difference (p > 0.05) in the fresh weight and dry weight of cucumber roots under the B1T1 and B1T2 treatments. of sucrose was not affected. Pathogen infestation and drought lead to a decrease in the rate of assimilation formation and the preferential distribution of assimilation to synthetic osmotic adjustment substances will inevitably affect the root-shoot ratio of plants. The influence of different nutrient solution irrigation treatments and downy mildew infection on the root-shoot ratio of cucumber is shown in Figure 4. As we can see from Figure 4, the root-shoot ratio of cucumber showed different responses under different nutrient solution irrigation treatments and downy mildew infection. The root-shoot ratio of cucumber under the B1T1, B1T2, B1T3, B2T1, B2T2 and B2T3 treatments was 3.16%, 2.99%, 4.11%, 3.92%, 3.13% and 3.63%, respectively. The root-shoot ratio of cucumber was the highest under the B1T3 treatments. When plants are infected by pathogenic and water-deficit stress, the degree of inhibition of root growth is much lower than that of aboveground growth. Therefore, limited assimilation is preferentially distributed to the root system, resulting in an increase in root-shoot ratio [21,39]. Leaf water status is severely affected by pathogenic and water-deficit stress at the cellular levels and encompasses disruptions in plant metabolism. On the other hand, due to the destruction of the leaf cuticle and increase in cell membrane permeability, water loss from the infected

Conclusions
Six treatments, i.e., B1T1, B1T2, B1T3, B2T1, B2T2 and B2T3, with downy mildew infestation at two levels, B1 (disease infestation) and B2 (no disease infestation), along with three fertigation requirement levels, full fertigation T1, moderate nutrient solution deficit T2 and severe nutrient solution deficit T3, influenced crop growth, leaf gas-exchange, material accumulation and parameters of root systems of cucumber grown in a greenhouse. The leaf gas-exchange parameters were significantly increased under the CK (B2T1) treatment, and leaf photosynthesis rate, transpiration rate and stomatal conductance were significantly decreased under the B1T1 treatment. Leaf intercellular CO 2 concentration was significantly increased under the B1T1 treatment during the whole experiment period. Leaf photosynthesis rate, transpiration rate, intercellular CO 2 concentration and stomatal conductance were significantly decreased under the B1T2, B1T3, B2T2 and B2T3 treatments during the whole experiment period. Compared with treatment CK (B2T1), the plant height of cucumbers under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 11.41%, 19.05%, 27.48%, 7.55% and 10.62%, respectively; the stem diameter of cucumber plants under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 5.70%, 13.45%, 23.03%, 9.46% and 15.74%, respectively; and the leaf area of cucumber plants under the B1T1, B1T2, B1T3, B2T2 and B2T3 treatments decreased by 22.79%, 38.68%, 58.28%, 13.76% and 29.96%, respectively. The root-shoot ratio of cucumber showed different responses under different nutrient solution irrigation treatments and downy mildew infection. The root-shoot ratio of cucumber under the B1T1, B1T2, B1T3, B2T1, B2T2 and B2T3 treatments was 3.16%, 2.99%, 4.11%, 3.92%, 3.13% and 3.63%, respectively. The root-shoot ratio of cucumber was the highest under disease infestation and the severe nutrient solution deficit treatment.